Chapter 36 Viewgraphs AC Circuits

1. Chapter 36 Viewgraphs AC Circuits

2. Introduction Most currents and voltages vary in time. The presence of circuit elements like capacitors and inductors complicates the relation between currents and voltage when these depend on time. Resistive element -I&V proportional Reactive elements involves derivatives Voltage and current are not simply proportional for reactive elements. Ohm’s law does not apply.

3. R V0 L Three categories of time behavior Direct Current (DC) Voltages and currents are constants in time. Example: batteries - circuits driven by batteries 2. Transients Voltages and currents change in time after a switch is opened or closed. Changes diminish in time and stop if you wait long enough. S VL(t) t

4. 3. Alternating Current (AC). The voltages and currents continually change sinusoidally in time. frequency amplitude phase Examples: our power grid when it is on. f=60 Hz, V=110 V (RMS) audio signals communication signals Power in microwave ovens Power in MRI machines Real Life voltages involve DC, AC and Transients

5. http://www.tvtower.com/images/rf-spectrum.jpg

6. Power Supply: converts AC to DC Present inside almost all home electronics Inverter: converts DC to AC Plugs into cigarette lighter, charges laptop. Don’t run a hair straighter on one of these while driving in your car.

7. AC - Circuits First Rule of AC - Circuits - everything oscillates at the same frequency If a circuit is driven by a source with frequency w, and you wait for all transients to die out, the circuit will reach a state where every voltage and current is oscillating at the same frequency w. Often this is called a “steady state” even though every thing is oscillating. The problem then becomes: Find the amplitude and phase of each voltage and current.

8. Complicated circuit: Rs, Ls, and Cs Every voltage will be in the form Every current will be in the form Problem is to find the amplitudes and phases

9. Some general comments about circuits driven by a source with frequency w. All voltages and currents oscillate at the same frequency w. Amplitudes and phases of voltages and currents depend on source and Rs, Cs, Ls, and w. Amplitudes of voltages and currents are proportional to source voltage. Phases of voltages and currents do not depend on amplitude of source voltage. Shifting the phase of the source shifts the phase of all voltages and currents by the same amount.

10. Vs(t) R I(t) L Let’s do a “simple” example Can take: Current I, flows through both R and L Resistor Voltage Inductor Voltage Inductor voltage is 90 degrees out of phase with resistor voltage and current

11. I(t) t When I(t) is maximum VL(t) VL(t) is zero and decreasing VL leads I by p/2

12. Which could be true? Red is the voltage across an inductor, black is the current through that inductor Black is the voltage across an inductor, red is the current through that inductor neither of the above

13. Which could be true? Red is the voltage across a capacitor, black is the current through that capacitor Black is the voltage across a capacitor, red is the current through that capacitor neither of the above

14. Vs(t) R I(t) L Let’s do a “simple” example Can take: Current I, flows through both R and L Resistor Voltage Inductor Voltage Inductor voltage is 90 degrees out of phase with resistor voltage and current

15. I(t) t When I(t) is maximum VL(t) VL(t) is zero and decreasing VL leads I by p/2

16. Kirchoff’s Voltage Law: sum of voltages around loop=0 for all t Find How to solve: 1. Use trigonometric identities 2. Collect terms multiplying and

17. After regrouping “Reactance” Can only be satisfied for all t if coefficients of cos and sin are separately equal. Solution:

18. R Resistance and Reactance equal Solution: Inductor voltage Resistor Voltage

19. Vs(t) R I(t) L Crossover network Low frequency Inductor is short All voltage appears across resistor High frequency Inductor is open All voltage appears across inductor

20. I R1 V0 R2 Recall for a moment when life was simple - DC circuits. Wouldn’t you do anything to get back to that simple way of analyzing circuits? Yes No What do you mean by anything?

21. Phasors: sinusoidal signals can be represented as vectors rotating in a plane. Later we will see that this is the complex plane Think of the time dependent voltage as the projection of the rotating vector on to the horizontal axis

22. What are the phasors for the voltages in our circuit? VR(t) and VL(t) form two sides of a right triangle, the hypotenuse is Vs(t)

23. The magnitude of the instantaneous value of the emf represented by this phasor is • constant. • increasing. • decreasing. • It’s not possible to tell without knowing t.

24. Bottom Line Everything you learned about DC circuits can be applied to AC circuits provided you do the following: Replace all voltages and currents by their complex phasor amplitudes. In practice this means putting a hat on each letter. Treat inductors as resistors with “resistance” jwL Treat capacitors as resistors with “resistance” 1/(jwC)

25. Phasors - a way of representing complex numbers Engineers use j Physicists and mathematicians use i Imaginary number Complex number X is the real part Y is the imaginary part Complex numbers follow the same rules of algebra as regular numbers Addition: -1 Multiplication:

26. A complex number is specified by two real numbers Instead of real and imaginary parts can give magnitude and phase Multiplying complex numbers - part 2 Magnitudes multiply Phases add

27. ex e0=1 1 x Exponential function Plot for real x But, what if x is imaginary? Let Then you can show: So:

28. Phasors Suppose I have an oscillating voltage I can write this as the real part of a complex number. Call this a complex amplitude or “phasor” In this class, hat means a complex # Magnitude of phasor gives peak amplitude of signal. Angle give phase of signal.

29. Multiplying by rotates the angle of the product by Remember: How to use in circuits: 1. Every voltage and current is written in phasor form:

30. 4. Cancel 2. Write every circuit law in terms of phasors: Example: Ohm’s Law VR(t) = R I(t) 3. Drop the Real. Real parts are equal and lets say imaginary parts are equal too. Why not? 5. The result is the same Ohm’s law we love, but with phasors!

31. 4. Cancel What about Inductors? Substitute in phasors Only t dependence 3. Drop the Real 5. The result is the same Ohm’s law we love, but with resistance replaced by

32. Vs(t) R I(t) L Back to our circuit KVL: Result: Recall DC circuit result:

33. Bottom Line Everything you learned about DC circuits can be applied to AC circuits provided you do the following: Replace all voltages and currents by their phasor amplitudes. In practice this means putting a hat on each letter. Treat inductors as resistors with “resistance” jwL Treat capacitors as resistors with “resistance” 1/(jwC)

34. Vs(t) R I(t) L C KVL RLC Circuit Current phasor Complex Impedance Magnitude of Impedance Phase of Impedance

35. Resonance: At what frequency is the amplitude of the current maximum? Complex Amplitude Current Amplitude Current is largest when this term is zero At resonance: Resonant frequency

36. How narrrow is the Resonance? Width of resonance determined by when these two are equal Quality Factor Decaying transient Quality factor determines rate of decay of transient envelope envelope

37. Vs(t) I(t) C L R What is the impedance of the parallel combination of an R, L, and C? A B C

38. Write currents and voltages in phasor form I(t) Vs(t) R Phasors for R-L circuit KVL L Write circuit equations for phasor amplitudes KVL: Result: Impedance

39. Impedance has a magnitude and phase Result: Impedance Resistor Voltage Inductor Voltage Note:

40. Power Dissipated in Resistor Current Instantaneous Power Average over time is 1/2 Average Power

41. Root Mean Square (RMS) Voltage and Current Current Average Power Peak current What would be the equivalent DC current as far as average power is concerned? No pesky 2 Average Power What is the peak voltage for 110 V-AC- RMS? A: 156 V

42. Power Delivered to a Capacitor Voltage Current Instantaneous Power Average Power

43. Rank in order, from largest to smallest, the cross-over frequencies of these four circuits.

44. A series RLC circuit has VC = 5.0 V, VR = 7.0 V, and VL = 9.0 V. Is the frequency above, below or equal to the resonance frequency? • Above the resonance frequency • Below the resonance frequency • Equal to the resonance frequency

45. The emf and the current in a series RLC circuit oscillate as shown. Which of the following would increase the rate at which energy is supplied to the circuit? • Decrease ε0 • Increase C • Increase L • Decrease L